The present invention relates to a hydraulic power booster, in particular for the actuation of a master cylinder in an automotive vehicle brake system, with a booster piston and a brake valve which is essentially arranged in parallel to the booster piston and by means of which pressure fluid is adapted to be metered into a booster chamber in dependence upon an actuating force, the booster chamber being confined by a pedal-operable piston rod and by the booster piston.
A device featuring the above characteristics is known from German printed and published patent application No. 31 08 908.9. In the known brake booster, a master cylinder piston is arranged in a first cylinder bore, forming one component together with a booster piston. A web connects the master cylinder piston and the booster piston together. A pedal-side front face of the booster piston confines a booster chamber formed within the housing and is positioned coaxially relative to a pedal-operable piston rod to which is hinged a lever arrangement for the actuation of a brake valve. By exerting a force on the piston rod and by correspondingly actuating the brake valve, a pressure is adapted to be metered into the booster chamber which is proportional to the actuating force and which displaces the booster piston and the master cylinder piston connected to the booster piston in the actuating direction of the brake booster. In this way a corresponding pressure will be built up in the working chamber of the master cylinder. Connected to the working chamber of the master cylinder are wheel brakes that are pressurized in this way. Thus on actuation of the brake pedal, a corresponding slowing-down of the vehicle will ensue.
Upon pressurization of the booster chamber, the piston rod coupled with the brake pedal will likewise be pressurized, so that a corresponding reaction force will be felt at the brake pedal which will provide feed-back information to the vehicle driver regarding the intensity of the braking action initiated by him.
Under certain operating conditions, however, it may be desirable to have a certain volume of fluid supplied to the wheel brakes even before a noticeable reaction force is felt at the brake pedal.